Allow multiple files to be fixed in parallel

[jwmatthews]

Kai allows multiple workers to run in parallel, when we run with run_demo.py we will send ~8 files at a time to migrate.

Let's test doing similar with the IDE and updates issues as they are found.
I attempted to do a few in parallel on April 4th, first thing I saw is there was no way to change between generated fixes.
Our approach seems to assume only one can occur at a time. I saw we could issue requests in parallel but I don't think our display logic is ready to handle this with allow multiple generate fixes to be ready and to switch between... probably related to #152

[jwmatthews]

@hhpatel14 this is needed, but it's not critical in terms of looking at functionality within next 2 weeks.